# Event callbacks

If a callback is registered for an event, it will be called when the event becomes active. To associate a callback with event one can pass a callable to either Event::__construct(), or Event::set(), or one of the factory methods like Event::timer().

An event callback should match the following prototype:

callback( mixed `$fd` = `null` , int `$what` = ?, mixed `$arg` = `null` ): void

`fd`

  The file descriptor, stream resource or socket associated with the event. For signal event `fd` is equal to the signal number.

`what`

  Bit mask of all events triggered.

`arg`

  User custom data.

Event::timer() expects the callback to match the following prototype:

callback( mixed `$arg` = `null` ): void

`arg`

  User custom data.

Event::signal() expects the callback to match the following prototype:

callback( int `$signum` = ?, mixed `$arg` = `null` ): void

`signum`

  The number of the triggered signal(e.g. `SIGTERM` ).

`arg`

  User custom data.

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